1. Field of the Invention
The present invention relates to an off-track detector employed for an optical information recording/regenerating apparatus, which records or regenerates information in or from an optical recording medium while controlling tracking by irradiating a light beam onto the optical recording medium, and the optical information recording/regenerating apparatus. More particularly, this invention is concerned with an off-track detector, which when off-track occurs during recording or regeneration, detects the off-track, and an optical information recording/regenerating apparatus.
2. Description of the Related Art
In recent years, various proposals have been made for an optical information recording/regenerating apparatus (hereinafter, optical disk unit) in which a light beam is converged and irradiated to an optical information recording medium (hereinafter, optical disk) to form a pit array or the like on the optical disk and thus record information optically, or light returned from a recorded pit array is received to regenerate recorded information.
In the foregoing optical disk unit, numerous tracks in which information is recorded are laid out on the optical disk so that information can be recorded with high density. For recording or regenerating information, tracking servo control is performed in which a tracking error signal is detected in the light reflected from or transmitted by the optical disk, and then a light beam is locked on to a track.
In a conventional optical disk unit in which the foregoing tracking error signal is used to determine whether a displacement of a light beam from a track on an optical disk exceeds a specified value. If the displacement exceeds the specified value, off-track is identified and off-track processing is performed. In the off-track processing, supposing data were being recorded on an optical disk, modulation would be stopped to decrease the light-emitting level to the level for regeneration and eventually prevent destruction of the optical disk. The optical disk would then be replaced with a new one if necessary. Supposing data were being regenerated, retry would be executed: a regenerative beam is returned to a regeneration start point, and regeneration is resumed with tracking controlled. As for off-track detection, a proposal has been made for an apparatus that will not be very responsive to a flaw on an optical disk. For example, an apparatus disclosed in Japanese Patent Laid-Open No. 63-7522 does not identify off-track when an output duration of a signal representing an off-track state is a specified time.
For an optical disk unit, various proposals have been made in recent years. Herein, the rotational frequency of an optical disk is made higher by shortening a recording time and raising a transfer rate. This kind of optical disk unit includes two recording/regenerating facilities: one facility permits information recording or regeneration at the conventional rotational frequency so as to provide compatibility with conventional optical disks, and the other facility permits information recording or regeneration at a rotational frequency that is, for example, twice the conventional rotational frequency. A servo system for this optical disk unit is designed to have a frequency characteristic compatible with an optical disk that is rotated at a relatively low rotational frequency of 1800 rpm.
In the conventional optical disk having two recording/regenerating facilities for recording or regenerating information at the conventional rotational frequency and the rotational frequency that is twice the conventional one. If an optical disk has a defect, a tracking error signal becomes larger in voltage value. One of the reasons is that when the rotational frequency is, for example, doubled, the eccentric acceleration in a radial direction is squared. The other reason is that as mentioned above, the servo system is designed to comply with an optical disk that is rotated at a relatively low rotational frequency. This servo system permits unsatisfactory tracking, which increases a residual of a light beam from a track on the optical disk. Consequently, the tracking error signal becomes larger in voltage value.
In a prior art method of determining occurrence for off-track by comparing a tracking error signal with a specified value, although a defect of an optical disk is recognized as a value smaller than a value specified in a comparator when the optical disk is rotated at a relatively low rotational frequency, the defect is detected as a value exceeding the value specified in the comparator when the optical disk is rotated at a rotational frequency that is twice the relatively low rotational frequency. Consequently, off-track is detected incorrectly. This incorrect detection brings about a problem that retry is repeated or a transfer rate decreases.